1. Field of the Invention
The invention relates to computer systems, and more particularly, to a circuit for holding the processor should a request be made prior to an allowable time.
2. Discussion of the Related Art
Personal computers are getting steadily more powerful as microprocessor manufacturers develop ever faster and more powerful devices. Fast memory techniques, such as caching, are utilized to allow the microprocessors to operate at the maximum possible speed.
Some operations in the personal computer take a minimum fixed amount of time to occur and are not being changed as microprocessor speeds increase. For example, a standard floppy disk controller chip is used in personal computers compatible with the IBM PC/AT manufactured by International Business Machines Corporation (IBM). One manufacturer of the standard floppy disk controller is NEC Electronics, Inc., which designates its part the .mu.PD765A (764). At time it is necessary to reset the floppy disk controller, either at system reset time or by software command. The 765 provides only a hardware reset input and does not itself provide a software reset command. To resolve this problem, a flip-flop which can be accessed by the processor is provided, the output of the flip-flop being connected to the reset input of the 765. The flip-flop is cleared by the system reset signal.
A problem arises because the reset signal to the 765 must be kept high for at least 3.5 microseconds according to product specifications. This time was not a problem in previous computer systems which used operating systems such as OS/2 by Microsoft Corporation and IBM. In OS/2, a portion of the software performs a software reset of the floppy disk controller. A zero value is written to the desired location, bit 2 of the byte located at either input/output (I/O) port 3F2 or 372 for IBM compatible systems. A short series of operations is performed and then a one value is written to the location to clear the reset and enable the floppy disk controller. In previous systems the time of execution of the various software operations between the and clearing setting of the location was sufficient to guarantee that the 765 was reset. However, as processor speeds increase, for example to 33 MHz for the 80386 system produced by Intel Corporation, the instruction sequence is performed in less than 3.5 microseconds, times of 1.6 to 2.0 microseconds having been measured for a 33 MHz 80386 having a cache memory and paged main memory. Thus the 765 cannot be reliably reset using standard software such as OS/2 operating in systems using high speed microprocessors.